1. Field of the Invention
The current invention relates to a method of realising reflectors of electromagnetic radiation in semiconductor substrates upon which high quality epitaxy can be grown. These substrates may be used to produce a resonant cavity for electromagnetic radiation: such cavities are of importance for applications such as laser etalons and infrared photon detectors.
2. Discussion of Prior Art
Infrared detectors are of importance for both civilian and military applications. Where radiation is transmitted through the atmosphere the useful wavelength ranges are limited to [3-5].times.10.sup.-6 m and [7.5-14].times.10.sup.-6 m by atmospheric absorption. For military applications the long wavelength range may be further limited to about [7.5-9].times.10.sup.-6 m by filters designed to avoid dazzle by CO.sub.2 lasers. Focal plane arrays (FPAs) with many individual detector elements are used for imaging in both these ranges. Several technologies are available for FPAs in the [3-5].times.10.sup.-6 m range eg CdHgTe (CMT) or InSb hybridised to Si readout circuits, or monolithic arrays. The technology is less well developed in the [7.5-14].times.10.sup.-6 m range. CMT is difficult to grow with the uniformity required for long wavelength infrared (LWIR) FPAs and requires cold bonding to the Si readout circuit which makes fabrication more difficult. Multi Quantum Well (MQW) detectors using GaAs/AlGaAs have progressed to 128.times.128 arrays using solder bump technology to hybridise to a Si readout chip but with unknown yield for larger arrays (B. F. Levine. J. Applied Physics 74 R1 (1993)). High performance FPAS are typically operated at temperatures close to that of liquid nitrogen. The thermal mismatch between detector and readout chips limits the size of hybrid arrays using compound semiconductor detectors on Si readout circuits. There is no successful Si monolithic LWIR FPA technology; IrSi, which has a long wavelength infrared response has low quantum efficiency and therefore very low operating temperature.
At present LWIR technologies are being developed which are compatible with the Si readout circuits to avoid limitations inherent in the hybrid approach using compound semiconductor detectors. Of these the use of pseudomorphic heterostructures formed from silicon-germanium alloys epitaxially grown on Si substrates (SiGe/Si) is the most promising. Pseudomorphic SiGe/Si has the advantage of allowing a very high degree of uniformity over the Si substrate wafer which minimises fixed pattern noise in the FPA and favours high manufacturing yield.
LWIR photon detectors made from SiGe/Si MQWs are limited by low responsivity and by dark current thermal noise. Devices must be operated at lower temperatures to give acceptable signal/noise ratios. This restricts the utility and increases the cost of the imaging system. An increase in the quantum efficiency for absorption of incident radiation should improve the signal/noise ratio and so allow operation at higher temperature.
The quantum efficiency of detectors at shorter wavelengths (&lt;2.times.10.sup.-6 m) has been enhanced using resonant cavities (see eg R Kuchibhotla, J Campbell. J C Bean, L Peticolas and R Hull, Appl. Phys. Lett 62 2215 (1993)). Resonant cavities produce localised regions of high electric field. Where absorption is dependent on the electric field the quantum efficiency can be enhanced by locating the absorbing region of a device in a resonant cavity.
To produce such a resonant cavity, the incident radiation wavelength .lambda., is confined between two reflectors. Electric field enhancement in the cavity increases as the reflectances of the confining mirrors at wavelength .lambda. increase. For a cavity of width L.sub.c and refractive index n, light confined in the cavity resonates at wavelengths determined by optical thickness nL.sub.c /.lambda., and the phase changes at the confining mirrors. Hence the cavity width, as well as the wavelength-dependence of the reflectors, must be chosen carefully to match .lambda.. Provided the reflectors and absorption region are sufficiently broad-band, resonances at more than one wavelength may be used.
The reflectors used in resonant cavities are often dielectric or semiconductor stacks which consist of a large number of pairs of layers with refractive indices n.sub.1 (higher than n for the cavity) and n.sub.2 (lower than n for the cavity) and with individual layer thicknesses .lambda./4n.sub.i (where i=1 or 2). An important property of such reflectors is their high transmission of incident light to allow penetration of the stacks and the cavity. Use of such Bragg reflectors for LWIR applications is limited, however, by difficulties in producing high quality pairs of thick layers to match the large wavelength. When one of the layers is strained, as for SiGe/Si, defects are introduced by strain relaxation in thick layers which reduce the efficiency of the Bragg reflector.
Semiconductor resonant structures are often grown epitaxially on single crystal substrates. A key requirement is then for a high efficiency reflector buried below the epitaxial active region of the device. Since epitaxial Bragg reflectors are unsuitable for LWIR applications, alternative types of buried reflector must be sought while maintaining the suitability of the substrate to produce high quality epitaxial structures. The reflector on top of the active region can simply be the semiconductor/air interface, or other layers grown epitaxially or deposited after epitaxy.
Single dielectric layers of SiO.sub.2, which have a refractive index of about 1.45 over a range of infrared wavelengths (whereas Si has a refractive index of about 3.45 in the infrared), have been suggested and demonstrated as buried reflectors in Si at shorter wavelengths (eg V P Kesan, P G May. F K LeGoues and S S Iyer, J. Cryst. Growth. 111 936 (1991); D K Nayak, N Usami, S Fukatsu and Y Shiraki, Appl. Phys. Lett. 64, 2373 (1994)). SIMOX wafers (separation by implantation of oxygen) were used as substrates. The high quality of Si overlying the oxide allows epitaxial overgrowth of Si and SiGe epilayers. The thickness of buried SiO.sub.2 that can be produced in SIMOX wafers is, however, limited.
An alternative technology for the production of high quality buried oxides in Si is that of bond-and-etch silicon on insulator (BESOI), where limitations on thickness are less severe. This is important at longer wavelengths where the thicknesses of SiO.sub.2 in the microns may be necessary to produce the required reflectances.
The weak absorption in SiO.sub.2 at infrared wavelengths shorter than 7.5.times.10.sup.-6 m results in a wavelength dependent reflectance for a single layer which exhibits maxima and minima at wavelengths determined by the SiO.sub.2 thickness. However, SiO.sub.2 exhibits strong phonon absorption bands at about 9.2.times.10.sup.-6 m and about 21.5.times.10.sup.-6 m. It is a property of such absorption bands that, in a wavelength region on the short wavelength side of each peak, the refractive index shows a local minimum. As a result of this property, in these spectral regions the reflectance of a single thick SiO.sub.2 layer buried in a Si ambient is enhanced compared with wavelength ranges of transparency removed from the absorption bands. In addition, in these spectral regions the reflectance is only weakly dependent on wavelength, and in the important wavelength region [7.5-9].times.10.sup.-6 m the reflectance is controlled by the oxide thickness, becoming almost independent of oxide thickness for layers &gt;1.5.times.10.sup.-6 m thick. This contrasts with the properties in the shorter wavelength range of dielectric transparency, where the reflectance is a strong function of both thickness and wavelength.
In principle, wafer bonding technology allows use of other buried dielectric layers having absorption bands, and enhanced reflectance, at different wavelengths. Similarly, a resonant cavity using a buried dielectric reflector and a different semiconductor material might be made by bonding that material to a Si wafer with the dielectric on its surface.
BESOI substrates have important advantages for making resonant cavity devices in the [7.5-9].times.10.sup.-6 m wavelength region. First, a thick (&gt;1.5.times.10.sup.-6 m) oxide layer can be used as the back reflector in the device, giving high reflectance. Secondly, the reflectance of a thick layer is not very sensitive to variations in thickness. Thirdly, the reflectance is reasonably constant over the wavelength region so that strong resonance can be observed even if the cavity width differs slightly from the design value, L.sub.c. As a result, the device performance is more tolerant to variations in both oxide thickness and cavity width than would be the case at shorter wavelengths.
Buried layers of SiO.sub.2 may also be used as front reflectors with light incident on the Si/SiO.sub.2 layer from the substrate side of the cavity. In this case the useful wavelength region of enhanced reflectance near absorption bands will be reduced by absorption of incident light in the SiO.sub.2 layer, for which the optimum thickness will be less than for oxide used as a back reflector. Anti-reflection coatings may be used to increase the light entering the substrate.
S Fukatsu, D K Nayak and Y Shiraki, in Applied Physics Letters. 65, 3039 (1994), report the use of single layers of SiO.sub.2 to provide reflectors for resonant cavities at wavelengths below 2.times.10.sup.-6 m. However, no reference is made to the spectral dependence of the refractive index, n, and extinction coefficient, k. FIG. 2 of this reference shows the reflectance of SiGe/Si epilayers over a single SiO.sub.2 layer buried in a Si ambient.
The wavelength dependence of the reflectance exhibits an oscillatory behavior corresponding to two resonances, one in the overlying SiGe/Si layers and one in the SiO.sub.2 reflector itself. The latter resonance occurs due to the low value of extinction coefficient in the SiO.sub.2 layer which results in light being reflected from the lower SiO.sub.2 /Si interface to give interference in the SiO.sub.2 layer and so a strong wavelength dependence of the layer's reflectance.
Fukatsu et al explain their reflectance results using Si and SiO.sub.2 refractive indices of n.sub.Si =3.45 and n.sub.SiO2 =1.45 respectively and by assuming low absorption (i.e. k is very small). Extending their model to longer wavelengths predicts a continuing oscillatory behavior of the reflectance. In the important wavelength region [7.5-9].times.10.sup.-6 m their model fails to predict the enhanced magnitude and wavelength-and thickness insensitivities of the reflectance which occur near regions of high k values and which are exploited in the present invention.